(1) Field of the Invention
The present invention relates to a novel resin which can suitably be used as any of optical materials such as plastic lenses, prisms, optical fibers, information recording substrates and filters, among others, as a material for plastic lenses of spectacles.
(2) Description of the Prior Art
In recent years, many plastic materials have been used as various kinds of optical materials, particularly as spectacle lenses, because they are light-weight, tough and easily dyeable. The optical materials, particularly the spectacle lenses are required to possess a low specific gravity as well as a high refractive index and a high Abbe's number as optical performances, and a high heat resistance and a high strength as physical performances. The high refractive index permits the decrease in a lens thickness, and the high Abbe's number can reduce a chromatic aberation of the lenses. In addition, the high heat resistance and the high strength facilitate the fabrication of the lenses, and they are important from the viewpoint of safety.
As conventional materials having the high refractive index, there have been suggested thermosetting optical materials having thiourethane structures obtained by reactions of polythiol compounds with polyisocyanate compounds (Japanese Patent Publication No. 58489/1992 and Japanese Patent Application Laid-open No. 148340/1993). In addition, other techniques, by which an epoxy resin or an episulfide resin is polymerized with a compound having two or more functional groups to obtain lenses, have also been suggested in Japanese Patent Application Laid-open Nos. 10 98615/1989 and 81320/1991 as well as WO 8910575. However, it is natural that a higher refractive index should be desirable. On the other hand, another important performance required for the optical material is that a chromatic aberation is low. Since the higher the Abbe's number is, the lower the chromatic aberation is, i.e., the better the chromatic aberation is, a material having the high Abbe's number is desired. That is to say, the simultaneous realization of the high refractive index and the high Abbe's number is also desired.
However, the Abbe's number usually tends to lower with an increase in the refractive index, and in a plastic material obtained from a conventional compound as a starting material, the Abbe's number is in the range of about 50 to 55 in the case that the refractive index is in the range of 1.50 to 1.55; it is 40 in the case of a refractive index of 1.60; it is about 31 which is a limit value in the case of a refractive index of 1.66; and it is about 30 or less in the case that it is forcedly attempted to achieve a refractive index of 1.7, and material having such a low Abbe's number is not practical any more.
In addition, in the case of the conventional technique, particularly in the case of a thiourethane material or the like, in order to express the high refractive index, it is necessary to increase the molecular weight of a raw material sulfur compound, so that a crosslink density deteriorates. Moreover, in order to express the high Abbe's number, it is necessary to increase an alkyl group content, so that the stiffness of the molecule constituting the raw material compound deteriorates, with the result that the heat resistance deteriorates inconveniently. That is to say, in the optical materials obtained from episulfide compounds, polythiol compounds and isocyanate compounds by the conventional technique, the increase in the refractive index is limited, and what is worse, the increase in the refractive index leads to the deterioration of the Abbe's number. In consequence, a suitable balance between the sufficiently high refractive index and refractive index has not been obtained so far.
For the purpose of solving the above-mentioned problems, the present inventors have found novel sulfur-containing compounds having an episulfide structure which permits the production of optical materials having a small thickness and a low chromatic aberation, and there have already been filed patent applications regarding the novel compounds (Japanese Patent Application Nos. 214631/1996 and 5797/1997). However, the optical materials obtained by polymerizing the sulfur-containing compounds cannot have a sufficiently satisfactory oxidation resistance, so that they tend to easily discolor during a long-term storage and in a process requiring a heat treatment.